1. Field of the Invention
The present invention relates to electrical test equipment and, more particularly, to a digital ohmmeter including apparatus for independently and digitally indicating electrical continuity.
2. Description of the Prior Art
Recent years have evidenced a rapid transformation of test instruments from ones having analog displays to ones having digital displays. Such instruments include voltmeters, ammeters, ohmmeters, and multimeters including all of the above functions. Test instruments having analog displays utilize meters, commonly a D'Arsonval movement, for providing a quantitative indication of the value of the unknown input. Digital instruments perform the same function, but display the output in digital form, commonly using a liquid crystal display.
A conventional digital test instrument such as an ohmmeter includes a pair of input terminals adapted to be connected across a circuit element of unknown resistance, a reference current of voltage source operatively coupled to the input terminals so that an analog voltage appears across the input terminals which is proportional to the resistance of the circuit element, a filter connected to one of the input terminals for filtering noise in the analog voltage, an analog-to-digital converter having an input connected to the filter and an output, a digital display, and driver cirucit means connected between the output of the analog-to-digital converter and the input of the digital display.
While digital test instruments of the type previously described are generally regarded as being quantitatively precise and have, therefore, acquired a significant share of the electrical test equipment market, their analog counterparts show significant advantage in applications where trend, rather than absolute, information is desired. An example of this advantage is in continuity measurement. In such circumstances, an analog ohmmeter can be used to quickly determine if electrical continuity exists in a circuit. Often, the user is less interested in the absolute value of conduction that he is in measurement speed.
For instance, a telephone service technician might utilize an analog ohmmeter to quickly check for a shorted wire within a cable by simply brushing the meter probe along the cable's connection points. If a short were present, it would be indicated by a momentary deflection of the meter. If the test points are closely spaced, the practiced user could check a multiple conductor cable in less than a second.
Digital ohmmeters, on the other hand, are poorly suited to perform such tests. They exhibit an inherent delay of from 0.5 to 1.5 seconds, resulting from the precise analog-to-digital conversion process, which makes rapid continuity indication all but impossible. Some digital instrument manufacturers, realizing this difficulty, have incorporated an ancillary electro-mechanical meter in their product to achieve the capability for a rapid continuity indication. Unfortunately, such an addition exacts a high cost for the indicator and its attendant drive circuitry, as well as requiring additional panel space for the meter. Also, since an electro-mechanical meter is often the only delicate component incorporated in the instrument, its inclusion tends to compromise instrument ruggedness and reliability.
Other instrument manufacturers have provided a separate electrical continuity tester capable of producing a digital output. Unfortunately, the user now must purchase and use two separate instruments and this is not only inconvenient but costly.